In eukaryotic cells DNA is packaged with histones to form chromatin. Approximately 150 base pairs of DNA are wrapped twice around an octamer of histones (two each of histones 2A, 2B, 3, and 4) to form a nucleosome, the basic unit of chromatin. Changes in the ordered structure of chromatin can lead to alterations in transcription of associated genes. This process is highly controlled because changes in gene expression patterns can profoundly affect fundamental cellular processes such as differentiation, proliferation, and apoptosis. Control of changes in chromatin structure (and hence of transcription) is mediated by covalent modifications to histones, most notably of their N-terminal tails. These modifications are often referred to as epigenetic because they can lead to heritable changes in gene expression, but do not affect the sequence of the DNA itself. Covalent modifications (for example, methylation, acetylation, phosphorylation, and ubiquitination) of the side chains of amino acids are enzymatically mediated.
The selective addition of methyl groups to specific amino acid sites on histones is controlled by the action of a unique family of enzymes known as histone methyltransferases (HMTs). The level of expression of a particular gene is influenced by the presence or absence of a methyl group at a relevant histone site. The specific effect of a methyl group at a particular histone site persists until the methyl group is removed by a histone demethylase, or until the modified histone is replaced through nucleosome turnover. In a like manner, other enzyme classes can decorate DNA and histones with other chemical species, and still other enzymes can remove these species to provide temporal control of gene expression.
The orchestrated collection of biochemical systems behind transcriptional regulation must be tightly controlled in order for cell growth and differentiation to proceed optimally. Disease states result when these controls are disrupted by aberrant expression and/or activity of the enzymes responsible for DNA and histone modification. In human cancers, for example, there is a growing body of evidence to suggest that dysregulated epigenetic enzyme activity contributes to the uncontrolled cell proliferation associated with cancer as well as other cancer-relevant phenotypes such as enhanced cell migration and invasion. Beyond cancer, there is growing evidence for a role of epigenetic enzymes in a number of other human diseases, including metabolic diseases (such as diabetes), inflammatory diseases (such as Crohn's disease), neurodegenerative diseases (such as Alzheimer's disease) and cardiovascular diseases. Therefore, selectively modulating the aberrant action of epigenetic enzymes holds great promise for the treatment of a range of diseases.
Mixed lineage leukemia (MLL) is a genetically distinct form of acute leukemia that constitutes over 70% of infant leukemias and approximately 10% of adult acute myeloid leukemias (AML) (Hess, 2004; Krivtsov and Armstrong, 2007). MLL represents a particularly aggressive form of leukemia and patients with this disease generally have poor prognoses; these patients often suffer from early relapse after treatment with current chemotherapies. There is thus a great and present need for new treatment modalities for patients suffering with MLL.
A universal hallmark of MLL disease is a chromosomal translocation affecting the MLL gene on chromosome 11q23 (Hess, 2004; Krivtsov and Armstrong, 2007). Normally, the MLL gene encodes for a SET-domain histone methyltransferase that catalyzes the methylation of lysine 4 of histone H3 (H3K4) at specific gene loci (Milne et al., 2002; Nakamura et al., 2002). Gene localization is conferred by specific interactions with recognition elements within MLL, external to the SET-domain (Ayton et al., 2004; Slany et al., 1998; Zeleznik-Le et al., 1994). In the disease-linked translocations, the catalytic SET-domain is lost and the remaining MLL protein is fused to a variety of partners, including members of the AF and ENL family of proteins such as AF4, AF9, AF10 and ENL (Hess, 2004; Krivtsov and Armstrong, 2007; Slany, 2009). These fusion partners are capable of interacting directly, or indirectly, with another histone methyltransferase, DOT1L (Bitoun et al., 2007; Mohan et al., 2010; Mueller et al., 2007; Mueller et al., 2009; Okada et al., 2005; Park et al., 2010; Yokoyama et al., 2010; Zhang et al., 2006). As a result, translocation products retain gene-specific recognition elements within the remainder of the MLL protein, but also gain the ability to recruit DOT1L, to these locations (Monroe et al., 2010; Mueller et al., 2007; Mueller et al., 2009; Okada et al., 2005). DOT1L catalyzes the methylation of H3K79, a chromatin modification associated with actively transcribed genes (Feng et al., 2002; Steger et al., 2008). The ectopic H3K79 methylation that results from MLL fusion protein recruitment of DOT1L leads to enhanced expression of leukemogenic genes, including HOXA9 and MEIS1 (Guenther et al., 2008; Krivtsov et al., 2008; Milne et al., 2005; Monroe et al., 2010; Mueller et al., 2009; Okada et al., 2005; Thiel et al.). Hence, while DOT1L is not genetically altered in the disease per se, its mislocated enzymatic activity is a direct consequence of the chromosomal translocation affecting MLL patients; thus, DOT1L has been proposed to be a catalytic driver of leukemogenesis in this disease (Krivtsov et al., 2008; Monroe et al., 2010; Okada et al., 2005; Yokoyama et al., 2010). Further support for a pathogenic role of DOT1L in MLL comes from studies in model systems that demonstrate a requirement for DOT1L in propagating the transforming activity of MLL fusion proteins (Mueller et al., 2007; Okada et al., 2005).
As briefly summarized above, there is evidence to suggest that the enzymatic activity of DOT1L is critical to pathogenesis in MLL. Therefore, it has been proposed that inhibition of DOT1L may provide a pharmacologic basis for therapeutic intervention in this disease. Compound treatment results in selective, concentration-dependent killing of leukemia cells bearing the MLL-translocation without effect on non-MLL transformed cells. Gene expression analysis of inhibitor treated cells shows downregulation of genes aberrantly over expressed in MLL-rearranged leukemias and similarities with gene expression changes caused by genetic knockout of the Dot1L gene in a mouse model of MLL-AF9 leukemia.